Evolution of the conceptual hydrogeologic and ground-water flow model for Las Vegas Valley, Clark County, Nevada Geological Society of America Annual Meeting November 14, 2 David J. Donovan Southern Nevada Water Authority
Outline Model development Improvement in geologic understanding
Conceptual Model Development Hydrologic Parameters Location, timing and value of: Water levels Ground water production / injection Spring, creek flow Major and minor wash flow Water usage by bare-soil and pheatophytes Natural recharge Secondary recharge Interbasin flow
Conceptual Model Development Hydrogeologic Parameters Location, and value of: Transmissivity - Horizontal permeability Leakance - Vertical permeability Storativity - Ability to store water Spatial distribution determined by a combination of geology and hydrology Assigned value primarily determined by hydrologic analysis (aquifer tests / modeling)
1981 MSS Image (2,3,1)
Meters above mean sea level Topography of Las Vegas Valley > 5, meters of fill
Major structures and lithologies in Las Vegas Valley
Hydrologic flow system and Hydrogeologic setting Hydrologic flow system. Vertical exaggeration: 1 to 1 West Edge of Las Vegas Valley Hydrographic Basin Ground water flow directions To Pahrump Hydrogeologic cross-section. Vertical exaggeration: 4 to 1 12,5 SPRING MOUNTAINS 1, Las Vegas Boulevard (Strip) La Madre Mountain Las Vegas 7,5 Las Vegas Lower Red Rock Springs Wash Frenchman Mtn. 5, Paleozoic Alluvial Fan Carbonate Rocks 25 Mesozoic to Upper Cambrian Paleozoic? -2,5 & Transitional Mesozic Cratonal? Sliciclasitic Rocks Rocks -5, Rocks pre-cambrian -7,5 Basement? -1, Lower Paleozoic -12,5 Carbonates? -15, UTM meters east (NAD 27) East To Colorado River -5 3 2 1-1 -2-3 -4-5
High permeability area in Las Vegas Valley High permeability area
Maxey and Jameson cross-section Generalized geologic cross-section of Las Vegas Valley (m) 1 West East "Near-surface water" DISTRICT "Shallow aquifer" MAIN WELL FIELD "Middle aquifer" "Deep aquifer" (m) NEW WELLS 75 EXPLANATION SILTY CLAYS 5 5 INTERBEDDED SANDY GRAVELS CALICHE WITH SAND AND GRAVEL UD 25 CARBONATE BEDROCK UD 5 UD KILOMETERS Modified from Maxey and Jameson (1948) Vertical Exageration ~ 17.5 25
Concept of allostratigraphic units
Allostratigraphic units cross-section West B' B METERS 1 Sandy silt Interbedded Gravelly c-g deposits Sand Sandy c-g deposits Unknown East 2.5 CARBONATE Land Surface ROCKS KILOMETERS Location of D TO D' cross-section 75? "middle part" 5 "lower part" U U D Inferred Fault 25 CLASTIC ROCKS Vertical Exaggeration = 1X
Hydrostratigraphic units cross-section West B METERS 1 Carbonate Rocks EXPLANATION East Low permeability Moderate permeability (aquitard) (aquifer) Very high permeability Unknown (aquifer) 75 Land Surface B' 2.5 KILOMETERS Location of D TO D' cross-section 1X 2 1A 3A 1B 4A 2 3B 5 4 5 Clastic Rocks 4B U 6 6B? U D Inferred Fault 25 Vertical Exaggeration = 1X
Hydrostratigraphic unit fence Diagram B Low Permeability 1 1 2 3 4 5 W High Permeability Moderate Permeability D' 75 N Wells used to create fence diagram 1A 2 D 3A 1X S 75 1B 1A 2 3A 1X 2B 1X 2A 1B B' 4 Potentiometric Surface 4 5 3B 5 5 6 25 3B 4B 6B? 2 6? 6 4B 5 4 5 2 4 1 3 2 Area Of Investigation Boundary Elevation Meters above mean sea level 7 Potentiometric Surface 6 3 6 4 1 E Carbonate Bedrock Clastic Bedrock 1A 1B 1X 2 2A 2B Las Vegas 3A Springs 3B Aquifer 4 4A 4B 5 6 Duck Creek 6B Aquifer Las Vegas Wash Aquitard
Relationship between allostratigraphic and hydrostratigraphic units Increase in sorting Poorly sorted, cemented coarse-grained deposits Well sorted lightly cemented moderately coarse-grained deposits Locally cemented fine-grained deposits Increasing distance from source
Hydrostratigraphic Units Very Simplified diagram of the ground-water system in the central part of Las Vegas Valley shallow geochemical zone Las Vegas Wash Aquitard Las Vegas Springs Aquifer aquitard (confining unit) 25 5 75 primary aquifer for non-municipal wells 1 aquitard 125 15 175 primary aquifer for municipal wells Duck Creek Aquifer 2 225 25 275 aquitard aquifer? 3 325 35 375 aquitard 4 425 aquifer? 45
Model Layers - Gross Simplification Layer 1 - Las Vegas Wash Aquitard 1 2 3 Leakance Boundary - Las Vegas Creek Aquifer Plus Twin Lakes Aquitard Layer 2 - La Madre Mountain Aquifer Leakance Boundary - Unnamed Aquitard at the Top of the Duck Creek Aquifer Layer 3 - Duck Creek Aquifer
Transmissivity map Layer 1 8 Transmissivity Zones Range of Values: 1 to 5, ft2/d
Upper Leakance layer 8 Leakance Zones Range of Values: 1.E-8 to 1.6E-4 ft2/d
Transmissivity map Layer 2 11 Transmissivity Zones Range of Values: 5 to 3, ft2/d
Lower Leakance layer 6 Leakance Zones Range of Values: 1.E-8 to 1.E-3 ft2/d
Transmissivity map Layer 3 6 Transmissivity Zones Range of Values: 5 to 1, ft2/d
Table - Transmissivity code 1 2 3 4 5 6 7 8 9 1 11 12 13 14 Transmissivity Type Inactive Carbonate Rock Clastic Rock Igneous/Metamorphic Rock Cemented Alluvial Fan gravel U/F U/C Alluvial Fan Gravel/Sand Faulted Sands/Silts/Gravel Unfaulted/Undiff. Silts/Sands/Clays Local moderately permeable areas Very permeable areas N/A Faulted Carbonate Rock Faulted Clastic Rock Faulted Igneous/Metamorphic Rock Local odd unit Layer 1 11 12 13 1,5 2,5 1,1 1, 5, 5, Layer 2 Layer 3 551 252 53 1,4 8,5 6,6 37 12,6 3,9 1, 1 5 5 51 52 1,214
Table - Leakance code 1 2 3 4 5 6 7 8 9 1 11 12 13 14 Leakance Type Inactive Carbonate Rock Clastic Rock Igneous/Metamorphic Rock Cemented Alluvial Fan gravel Alluvial Fan Gravel/Sand Faulted Sands/Silts/Gravel Unfaulted/Undiff. Silts/Sands/Clays Local moderately permeable areas Very permeable areas N/A Faulted Carbonate Rock Faulted Clastic Rock Faulted Igneous/Metamorphic Rock Local odd unit L 1-2 L 2-3 1.1E-6 2.E-8 1.E-8 1.5E-6 1.6E-4 1.3E-5 1.E-7 1.5E-6 1.E-6 2.E-8 1.E-8 1.E-4 1.E-3 1.E-7
Conclusions Hydrologeologic models primarily test geologic concepts Geologic understanding has increased in the last two decades Hydrologic data is the compilation of causes and effects on the ground-water system Most observed ground-water system changes are a result of multiple causes
Artesian wells in Las Vegas Valley 1912 1998
Table - Storativity code 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 Storage Type Inactive Carbonate Rock Clastic Rock Igneous/Metamorphic Rock Cemented Alluvial Fan gravel Alluvial Fan Gravel/Sand Faulted Sands/Silts/Gravel Unfaulted/Undiff. Silts/Sands/Clays Local moderately permeable areas Very permeable areas N/A Faulted Carbonate Rock Faulted Clastic Rock Faulted Igneous/Metamorphic Rock Local odd unit Local odd unit 2 Layer 1 1.E-1 1.E-1 1.E-1 2.5E-1 1.5E-1 Layer 2 1.1E-2 1.2E-2 1.3E-2 1.4E-2 8.5E-3 1.6E-2 1.7E-3 1.1E-1 5.9E-2 1.11E-1 4.15E-2 Layer 3 1.1E-3 1.2E-3 1.3E-3 1.7E-3
Location of Las Vegas Valley
Allostratigraphic units Very Simplified diagram of the basin-fill in the central part of Las Vegas Valley shallow geochemical zone Tule Springs Alloformation (Las Vegas Fm.) silt gravel Lone Mountain Allogroup silt 5 75 1 125 15 gravel with caliche Paradise Valley Alloformation 25 175 2 225 25 silt 275 3 325 gravel 35 375 silt 4 gravel 45 425
Major lithic types: Las Vegas Valley